[1] Univ N Carolina, Dept Biol, Chapel Hill, NC 27599 USA
[2] Univ N Carolina, Curriculum Genet & Mol Biol, Chapel Hill, NC 27599 USA
[3] Stanford Univ, Sch Med, Howard Hughes Med Inst, Dept Biochem, Stanford, CA 94305 USA
[4] Univ Calif San Francisco, Dept Biochem & Biophys, San Francisco, CA 94143 USA
[5] NCI, Biochem Lab, NIH, Bethesda, MD 20892 USA